Injection molding of various members used in vehicle bodies and the like is well known. Typically, members are formed from thermoplastic material via injection molding and such members can include single-walled open and/or planar members, such as interior trim pieces, roof rails, etc.
For single-walled open members or planar members which must bear larger loads, such as roof rails, etc., the thermoplastic material is often reinforced with material such as glass fibers, which are added to the thermoplastic melt from which the member is to be molded.
However, generally such single-walled open members or planar members cannot bear structural loads, even when reinforced with glass fibers in the conventional manner. If such single-walled open members or planar members are required to bear structural loads, for example to support other components attached to them or to support their own attachment to a vehicle, metal inserts to carry the loads must be formed in the molded part or separate reinforcements such as metal plate gussets must be provided.
More recently, water-assist injection molding has been developed. Water assist injection molding is similar to gas assist injection molding with water replacing most of the gas which is injected into the molten plastic shot in the mold to form one or more hollow cavities within the finished molded part. The greater pressures which can be achieved with water, compared to gas, and the fact that water is incompressible, result in much better wall thickness uniformity than can be achieved with gas assist technologies.
Accordingly, none of the known technologies for injection molding can produce, at a reasonable cost, injection molded members with load carrying capabilities permitting the finished members to be used as structural members.